Stereospecific metallocenes can be characterized generally as coordination compounds incorporating cyclopentadienyl groups or derivatives thereof (which may be substituted or unsubstituted) coordinated with a transition metal. Various types of metallocenes are known in the art. They include bicyclic coordination compounds of the general formula:(Cp)2MeQn   (1)characterized by the isospecific metallocenes as described below and cyclopentadienyl fluorenyl compounds of the general formula:Cp Cp′MeQn   (2)characterized by the syndiospecific metallocenes as described below. In the aforementioned formulas the Me denotes a transition metal and Cp and Cp′ each denote a cyclopentadienyl group which can be either substituted or unsubstituted with Cp′ being different from Cp, Q is an alkyl or other hydrocarbyl or a halo group and n is a number within the range of 1-3. The cyclopentadienyl groups are in a stereorigid relationship normally provided by a bridged structure between the metallocene groups (not shown in Formulas (1) and (2) above) although stereorigidity can be provided through substituent groups which result in steric hindrance, as described, for example, in U.S. Pat. No. 5,243,002 to Razavi.
Isospecific and syndiospecific metallocene catalysts are useful in the stereospecific polymerization of monomers. Stereospecific structural relationships of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers from various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically unsaturated monomers such as C3+ alpha olefins such as propylene, 1-butene, 4-methyl-1-pentene, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g., styrene or vinyl toluene, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g., isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is of most significance in the production of isotactic or syndiotactic polypropylene and polybutene.
The structure of isotactic polypropylene can be described as one having the methyl groups attached to the tertiary carbon atoms of successive monomeric units falling on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows:
In FIG. 3 each vertical segment indicates a methyl group on the same side of the polymer backbone. Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad as shown above is . . . mmmm . . . with each “m” representing a “meso” dyad, or successive pairs of methyl groups on the same said of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene using the Fisher projection formula can be indicated by racemic dyads with the syndiotactic pentad rrrr shown as follows:
Here, the vertical segments again indicate methyl groups in the case of syndiotactic polypropylene, or other terminal groups, e.g. chloride, in the case of syndiotactic polyvinyl chloride, or phenyl groups in the case of syndiotactic polystyrene.
Syndiotactic polymers are semi-crystalline and, like the isotactic polymers, are largely insoluble in cold xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is non-crystalline and highly soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.